The three-way catalytic converter no longer satisfies the requirements as to the conversion of nitrogen oxide in the operating region of the combustion of lean air/fuel mixtures (.lambda.&gt;1). Here, NOx storage catalytic converters are used which store the nitrogen oxides emitted in lean engine operation. Stored nitrates are released and converted to nitrogen by the operation of the engine in the rich region (.lambda.&lt;1).
The use of NOx storage catalytic converters in this connection is disclosed, for example, in U.S. Pat. No. 5,473,887.
Statutory requirements provide for an on-board diagnosis of the vehicle components, which are relevant to toxic emissions, such as catalytic converters. In this connection, it is known, for example, from U.S. Pat. No. 3,969,932, to apply the signals of oxygen-sensitive exhaust gas probes, which are mounted forward and rearward of the catalytic converter, for evaluating a three-way catalytic converter. The known method is based upon the oxygen storage capability of an operational three-way catalytic converter. In this connection, U.S. Pat. No. 3,969,932 discloses a change of the air/fuel mixture composition from .lambda.=0.95 (rich, fuel-rich mixture, oxygen deficiency) to .lambda.=1.05 (lean, fuel-poor mixture, oxygen excess). The exhaust-gas sensor mounted forward of the catalytic converter reacts thereto virtually without delay. The oxygen storage locations of the catalytic converter are at first not occupied because of the oxygen deficiency in the exhaust gas, which is present for .lambda.=0.95. The oxygen storage locations are successively occupied after the switchover to oxygen excess forward of the catalytic converter. Accordingly, at first there continues to be an oxygen deficiency rearward of the catalytic converter. After a time span, which is dependent upon the oxygen storage capability of the catalytic converter, an oxygen excess also occurs rearward of the catalytic converter which triggers a change of the signal of the rearward exhaust-gas sensor. The time delay, that is, the phase shift between the reactions of both exhaust-gas sensors, is less with decreasing capability of oxygen storage of the catalytic converter and can therefore be used for evaluating the oxygen storage capability for diagnosing the catalytic converter.
This known method is not easily transferrable to a catalytic converter, which in addition to a storage capability for oxygen also has a storage capability for nitrous oxides. Catalytic converters of this kind can usually still store nitrogen oxides when their oxygen storage capability is already exhausted and an exhaust-gas sensor, which is mounted rearward of the catalytic converter, indicates an oxygen excess. The time delay between the reactions of both exhaust-gas sensors after a changeover from rich to lean mixture therefore supplies no statement for NOx storage catalytic converters as to their NOx storage capability.